1. Field of the Invention
The present invention relates to an apparatus for manufacturing a sintered body with high density, and more particularly to a structure of an apparatus for applying hot isostatic pressing.
2. Description of the Prior Arts
As a method of manufacturing a metallic or ceramic sintered body with high density by making use of metallic or ceramic powders as starting materials, a method is well known wherein those starting materials are firstly sintered into a sintered body, and then, hot isostatic pressing is applied to the sintered body. In advance of the sintering and the hot isostatic pressing, the starting materials are preprocessed to be formed into a compact. The method of the formation is generally classified into three types.
(a) Press forming method: Material powders are firstly put together with a dispersion medium into a ball mill, and then, agglomerated lumps thereof or original grains thereof are crushed, while mixed with the dispersion medium. Subsequently, to these crushed materials, a binder composed mainly of wax is added, and those materials are put into a spray drying process to extract the dispersion medium therefrom, thereby granulated powders being formed. The granulated powders thus obtained are press-formed, for example, by means of hydraulic forming, into a compact. This compact is degreased through a degreasing process. This degreasing process is carried out by a method wherein the binder and the dispersion medium included in the compact is evaporated or pyrolyzed, for example, by means of vacuum heating and to be removed outside in the form of gas.
(b) Casting method: Material powders together with a liquid dispersion medium are firstly put into a mixing tank equipped with a stirrer, and the material powders are mixed with the dispersion medium to be formed into slurry. The slurry is cast into a mold made of water-absorptive material such as gypsum and the dispersion medium is absorbed in the mold, to thereby give a feature of maintaining the shape to the slurry. Subsequently, the mold is demolded to obtain a compact. Another method is also known wherein a mold made of non-water-absorptive and well heat conductive material such as metal is cooled in advance and the slurry is cast into the mold to give the shape-maintaining feature to the slurry by means of freezing the dispersion medium. Subsequently, the mold is demolded to obtain a compact. In this method, no binder is used, and therefore, the degreasing process is needless, but, in stead, a dispersion medium existing in voids among grains constituting the compact is removed in the drying process following the casting step.
(c) Plastic forming method: Material powders together with a binder are put into a kneader to form pellets. The pellets thus kneaded are charged into a molding machine to form a compact with a predetermined shape. The compact is transferred to a degreasing process, which is ordinarily carried out by heating.
As clearly understood from the aforementioned, sintering and hot isostatic pressing thereafter cannot be applied to the compact until the compact has been prepared through a lot of series of the pretreating processes. In the hot isostatic pressing process, a sintered body formed from the compact is compressed by high pressure nitrogen or argon gas and the sintered body has come to be of the theoretical density or in the vicinity thereof. Thus, a sintered body with high density can be obtained.
The disadvantages pointed out of the aforementioned manufacturing methods are that three steps of degreasing or drying, sintering and hot isostatic pressing, each, are carried out, by independent process equipment, and therefore, transference of a compacted body is required every time the steps are shifted. This results in affecting unfavorably improvement of production efficiency, and being in danger of impairing quality of products because the compact is exposed to the air.
As a means for overcoming these difficulties, for example, in the technical journal "Metal Powder Report", July, 1983, P.404, an apparatus is disclosed, wherein the three steps of degreasing or drying, sintering and hot isostatic pressing can be carried out in vacuum, hydrogen or other gaseous atmospheres without handling a compacted body in a step-to-step transference, and this apparatus has been successfully applied to sintered hard alloy. This method can be applied to the press forming method mentioned in (a), because a mixed ratio of a binder in the pretreating process is so small that the degreasing step does not take so much time. But, this method is not applicable to both of the casting and the plastic forming methods mentioned in (b) and (c). This is because heating employed in the casting method requires 20 to 100 hours to remove a dispersion medium without producing cracks of the compact, and heating employed in the plastic forming method also needs 100 to 150 hours to degrease much amount of a binder used therefor. Resultantly, those two methods lower exceedingly an actual operation rate of a high investment cost apparatus capable of sintering and hot isostatic pressing. Consequently, the difficulties have partially solved, but still remain unsettled.